This invention generally relates to refrigeration, and particularly to heat pumps, and even more specifically to heat pumps designed for use in northern climates.
In a typical vapor compression refrigeration circuit, various components, including a compressor, two heat exchangers, and an expansion device, are arranged and operated to transfer heat from one location to another. For example, heat may be transferred from a room within a building to the outdoors, cooling the room. With a heat pump circuit, a reversing valve is provided to reverse refrigerant flow through portions of the circuit so that the circuit may transfer heat not only from a first location to a second, but also from the second location to the first. For example, a first heat exchanger may be located within an enclosure and a second heat exchanger may be located outside the enclosure. In a cooling mode, heat is transferred from the first heat exchanger to the second, cooling the enclosure. In a heating mode, heat is tranferred from the second heat exchanger to the first, heating the enclosure.
Heat pumps operate very effectively, efficiently, and economically under many conditions and, not surprisingly, have found considerable acceptance. However, in certain areas, for example in the northern regions of the United States, heat pumps have heretofore not met with much commercial success. A major reason for this is because the maximum heating load in a room or building in such areas is relatively large, while the typical cooling load is comparatively small; and difficulties have been encountered in providing a heat pump which will effectively satisfy the former and, at a different time, efficiently meet the latter. This difficulty is exacerbated with the common, air to air heat pump--wherein both heat exchangers of the heat pump are in direct heat transfer relation with ambient air--because the capacity of the heat pump to transfer heat from the outdoors to inside an enclosure decreases as the outdoor air temperature decreases. Hence, just as the heating load on the heat pump is increasing, the ability of the heat pump to meet that load is decreasing.
One basic design approach for resolving this difficulty is to provide a heat pump with two serially arranged compressors and with means for selectively conducting refrigerant vapor around one of the compressors to by-pass that compressor. When the demand on the heat pump is small or modest, only one compressor is activated and the other compressor is by-passed; but when the demand on the heat pump is large, both compressors are activated. With both compressors operating in series, the refrigerant vapor is compressed, and thus its temperature is increased, to a greater degree, increasing the capacity of the heat pump.
When two compressors are arranged in series in a heat pump as described above, several potential difficulties are created. One difficulty is to insure that refrigerant vapor is properly routed through the heat pump. That is, through both compressors in series when this is desired, and through only one of the compressors when this is desired. Another difficulty is to insure that, when both compressors operate in series, the refrigerant temperature does not become excessive--that is, reach a level where the refrigerant, or any lubricant mixed with the refrigerant, may chemically decompose. Naturally, it is desirable that the resolution of these difficulties be not only effective and reliable, but also, to the extent consistent with these considerations, simple and inexpensive.